1. Technical Field
The present invention generally pertains to magnetic storage devices and more particularly to magnetoresistive (MR) transducer head elements.
2. Background Art
Our modern society is heavily dependant upon computer systems for everyday activity. Computers are found in our homes, in business offices, and in most production and manufacturing environments. Most computer systems are controlled by a central processing unit (CPU) and have various types of memory storage components which can be used by the CPU to operate and perform the various functions for which it has been programmed.
Traditionally, computer system memory storage components have been classified as either main memory (primary or main storage) or secondary memory (secondary storage). Information in main memory may be accessed directly by the CPU. Information in secondary memory, however, must be loaded into main memory for the CPU to access this information. Main memory is typically relatively small, fast, and expensive when compared to secondary storage components. As a result, computer systems typically rely on large quantities of secondary storage to economically hold a large amount of information that the CPU may need to access.
Secondary storage is often provided in the form of a Direct Access Storage Device (DASD). Typical examples of DASDs include hard disk drives, tape drive subsystems, and Compact Disk Read Only Memory (CD-ROM) drive units. Even entry-level home computer systems will have approximately 850 megabytes to two gigabytes of secondary storage, usually in the form of a single hard disk drive unit. Many of the newer home computer systems will also include a CD-ROM drive as well. Computer systems used in larger business and commercial operations often utilize multiple DASD units, with hard disk drives and tape backup systems being very common.
A typical hard disk drive unit is composed of multiple circular storage platters mounted inside a housing. The storage platters have a coating of magnetic material with small regions that define binary digits (or bits) that may be polarized in either of two directions. These magnetic storage platters are used by the computer system to store information that may be needed by the CPU. In order to store data on the platters, small read/write heads are placed in close proximity to the surface of the storage platters while the platters turn. During a write operation, the write heads change the magnetic characteristics of the surface of the platter, thereby storing data received from the CPU on the platter. During a read operation, the read heads sense the differences in the magnetic characteristics of the surface of the platter and transmit the data read from the platter to the CPU. A tape drive operates in a very similar manner but the storage medium takes the form of a magnetic tape instead of a platter.
The read heads of a DASD unit are frequently manufactured from magnetoresistive (MR) transducer elements. MR read head performance is significantly affected by several factors, most notably, ambient temperature and bias current. The effects of ambient temperature and bias current will be considered individually.
For the purposes of this discussion, ambient temperature is to be considered the temperature inside the DASD enclosure that houses the MR transducer heads and magnetic media (e.g., platters in a hard disk drive). Ambient temperature has a measurable effect on the performance of the DASD. When the ambient temperature decreases, the performance of the DASD degrades and error rates increase. While all of the reasons behind this phenomenon are not completely understood, the effect is most likely due to a combination of multiple factors. These factors include increases in the distance between the MR transducer heads and the surface of the storage media, higher levels of media noise, and poorer signal recording which, in turn, decreases the signal amplitude.
The effects of bias current on MR transducer head performance are more directly measurable. The ability to read a signal from the storage media is, in part, a function of the amount of bias current supplied to the MR head. Signal sensitivity can be increased by increasing the amount of bias current supplied to the MR head. Therefore, increased bias current will generally produce an improved signal-to-noise ratio and will therefore result in lower error rates. Signal-to-noise ratio is the comparison between the amount of desired data signal and the amount of undesired background signal that the MR read head and the other DASD components process. The signal-to-noise ratio can be improved by either increasing the signal level or decreasing the noise level. However, simply increasing the bias current is not a complete solution to improving MR read head performance because excessive bias current can significantly and unnecessarily shorten the useful life span of the MR read head.
Bias current can adversely affect MR read head life in two different ways. First, application of bias current in excessive quantities can cause the MR element to overheat. If the current density reaches a high enough level, the MR element will actually burn out. This type of catastrophic failure is typically avoided by selecting a bias current level for the MR read head that will keep the MR read head from burning out over the entire operating temperature range of the MR read head.
Catastrophic failure, however, is not the most common cause of MR read head failure. The most common cause of MR read head failure is a phenomenon known as electromigration. Constant exposure to even normal operating levels of bias current will, over an extended period of time, change the molecular structure of the MR read head, thereby degrading the magnetic sensing capability of the MR read head.
In existing DASD units, the MR heads are typically analyzed to determine the range of their operating characteristics over temperature and bias current variations. The performance of MR heads fabricated on a given fabrication line may vary considerably due to process variations that cause different geometric features on the heads. To assure that even the MR read head with the worst-case geometric tolerances will have at least the minimum desired lifetime, a bias current is selected for all of the MR read heads that will keep the temperature of all MR read heads below predetermined threshold levels. This pessimistic approach provides the desired minimum lifetime for the DASD unit, but does so at the expense of driving all heads with a bias current that is selected based on the worst-case. Of course, MR heads that are in the nominal range of manufacturing tolerances could be driven with a higher bias current to boost their performance without exceeding the relevant temperature thresholds, but this higher bias current would significantly shorten the life of MR heads at the worst-case of expected manufacturing variations in the MR heads. As a result, the maximum bias current for all heads is typically set to equal the maximum bias current for the worst-case head. Without new ways to provide bias current to MR read heads, the overall performance of DASD storage devices will be limited.